Stub tuned circulator

ABSTRACT

A strip line Y-junction ferrite circulator is tuned by locating a stub at the junction between a pair of arms. Each stub acts to adjust the impedance presented at the input arm.

[ 1 June 27, 1972 I References Cited UNITED STATES PATENTS [54] STUB TUNED CIRCULATOR [72] Inventor: Kenneth L. Carr, Bedford, Mass.

M i wkm mma GJD 77925 66566 99999 11111 829 3 1 1 40546 7 921 ,55 59334 35 67 33233 M m u 1 7 h 9 n. m 1 n. l A M 7 M 7 h 9, U mmn m F M 1 am m R e e mam w k w A m A 1]] 3 2 .l. 7 2 2 [Ii mr m m wi mm a .m am M Mm mm [63] Continuation of Ser. No. 468,844, July 1, 1965, abandoned.

[57] ABSTRACT A strip line Y-junction ferrite circulator is tuned by locating a stub at the junction between a pair of arms. Each stub acts to adjust the impedance presented at the input arm.

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19 Claims, 8 Drawing Figures FERRITE PATENTEnJum m2 3,673,518

saw 1 or 2 INHERENTLY MATCHED Flei INVENTOR KENNETH L. CARR BY WM EW,

ATTORN EYS PATENTEDJUNZ'I m2 3. 673 5 1 8 SHEET 2 or 2 A I I f r l if I 1I i llll INVENTOR. KENNETH L. CARR I BY W ATTORNEYS STUB TUNED CIRCULATOR This application is a continuation of my co-pending application Ser. No. 468,844, filed July 1, I965, now abandoned.

This invention relates to the non-reciprocal transmission of electromagnetic wave energy and pertains to devices employing ferrites to control the transfer of microwave energy. The invention concerns ferrite circulators of the type in which the center conductor of the strip line has a plurality of arms connected to a common junction located between a pair of ferrite members.

In transferring wave energy, the conventional strip line Y- junction circulator presents impedance discontinuities in the transfer path which have deleterious effects upon the energy transmission and adversely affect the bandwidth of the device. To make the transfer path electrically smoother, dielectric rings are employed in the conventional strip line Y-junction circulator. The dielectric rings increase the bulk of the circulator and because the dielectric ring! are, in essence, a form of quarter wave lmpednnce transformer, the impedance lransl'or matinn is effective over a relatively restricted frequency band. That is, the impedance transformation effected by the dielectric rings is frequency sensitive and thereby restricts the bandwidth of the conventional strip line Y-junction circulator.

In my copending patent application Ser. No. 353,357, filed Mar. 20, 1964 now 0.8. Pat. No. 3,355,679, entitled Inherently Matched Wave Transmission Apparatus, there are disclosed improved Y-junction strip line circulators in which the conventional impedance matching dielectric rings are eliminated by adjusting the width of the center conductors of the strip lines forming the arms of the circulator or by adjusting the ground plane spacing in the vicinity of the ferrite medium. Those improved circulators are deemed to be inherently matched" devices because matching is achieved not by the use of impedance transformation apparatus, but rather is achieved by adjustments in the strip transmission line itself. Inherently matched circulators operate over a broader band of frequencies than can be accommodated by conventional Y-junction strip line circulators and perform better than or at least as well as the conventional devices. The performance and the bandwidth of the inherently matched circulators can be further improved by employing the invention here disclosed.

The invention resides in a strip line circulator in which the characteristic impedance of the arms is fixed at a suitable level by inherent matching and in which tuning stubs are employed at the junction. Each tuning stub is located at the junction between a pair of arms and acts to adjust the impedance presented at the input arm; that is, the impedance of each arm of the circulator can be independently adjusted by tuning the stub at the junction which is located between that arm and an adjacent arm.

The invention, both as to its construction and its manner of operation, can be better apprehended from the exposition which follows when it is considered in conjunction with the accompanying drawings in which:

FIG. 1 depicts a prior art" strip line symmetrical junction circulator and shows the conventional dielectric matching rings employed in such structures,

FIG. 2 illustrates a strip line circulator in which inherent matching is obtained by adjustment in the width of the lines center conductor,

FIG. 3 is a plan view of the junction in the FIG. 2 circulator,

FIG. 4 illustrates a strip line circular in which inherent matching is achieved by adjustment of the ground plane spacmg,

FIG. 5 depicts, in a partially exploded view, a circulator embodying the invention,

FIG. 6 is a plan view of the junction in the FIG. 5 circulator and shows the arrangement of tuning stubs between the arms,

FIG. 7 shows the relation of the tuning stubs to the radio frequency field occurring in the ferrite area when a signal is transmitted from one port to another,

FIG. 8 is a partially exploded view of an embodiment of the invention employing a change in ground plane spacing to maintain the characteristic impedance of the arms at an appropriate level.

The conventional construction of a strip line Y-junction circulator is illustrated in FIG. 1. The ground planes of the strip line are constituted by plates 1 and 2 and between those plates is disposed a center conductor having three arms 3, 4, 5 meeting at a central junction 6. To fix the ground plane plates so that they are parallel and separated, the plates are secured to three spacers 7, 8, and 9. The spacers are electrically conductive to act as short circuits which maintain the ground planes at the same electrical potential. Each spacer is, in addition, electrically connected to the outer conductor of a coaxial connector which has its center conductor insulated from and extending through the spacer into electrical connection with one of the arms of the strip line's center conductor. The arm 3, for example, is joined to the center conductor of the coaxial connector 10 whose outer conductor is joined to spacer 7. Similarly, arm 4 is connected to the center conductor of coaxial connector II and arm 5 is joined to the center conductor of coaxial connector II.

The central junction 6, at which the three arms meet, is disposed between a pair of ferrite discs, only one of which, the disc 13, is in view in FIG. 1. Each disc is fitted within a dielectric ring, 14 or 15, and the dielectric and ferrite assemblages fill the spaces between the central conductor and ground plane plates above and below the assemblages. In the operation of the circulator, the ferrite discs are situated in a magnetic field established by a magnet having its pole pieces accommodated within depressions, such as the depression 16, in the ground plane plates. The bandwidth over which the circulator operates satisfactorily is, as is known, dependent upon the degree of matching that is achieved, the size of the ferrite discs, and the intensity of the magnetic field. In order to alleviate the impedance mismatch caused by the ferrite at the junction, dielectric loading of the strip line is employed to obtain an electrically smoother path. The conventional manner of dielectric loading is the placement of a dielectric ring around the ferrite disc, as illustrated in FIG. 1. This dielectric matching technique extends somewhat the bandwidth over which satisfactory operation of the circulator is obtained.

The wave energy in proceeding through the conventional circulator propagates through three dielectric media, viz., air, the material constituting the dielectric rings, and ferrite; that is, wave energy in proceeding from an input arm to an output arm sees, in consecutive order, an air filled strip transmission line, a dielectrically loaded strip line, a ferrite filled line, a dielectrically loaded line, and an air filled line.

The characteristic impedance Z, of a strip transmission line is given, approximately, by the equation where e is the dielectric constant of the wave transmission medium, b is the spacing between ground planes, and Wis the width of the center conductor. The equation assumes that the thickness of the center conductor is negligible; that is, that the thickness of the center conductor is vanishingly small. From the equation it is evident that an increase in the dielectric constant e of the wave transmission medium causes a decrease in the characteristic impedance of the line where b and W are maintained unchanged.

In the conventional strip line circulator, the coaxial connectors usually are standardized to have an impedance of 50 ohms. The characteristic impedance of the air filled portion of the strip line is selected to match the characteristic impedance of the coaxial line in order to avoid an impedance discontinuity. As the spacing of the ground planes is fixed, the width W of the strip lines center conductor is determined by the air filled line's characteristic impedance. The width W of the center conductor is maintained unchanged in the dielectric filled portion of the strip line and because the dielectric constant of the rings may be fourfold greater than the dielectric constant of air, the characteristic impedance of the dielectric filled strip line drops to a value that is about half of the characteristic impedance of the air filled line.

My copending patent application Ser. No. 353, 357, filed Mar. 20, 1964 now U.S. Pat. No. 3,355,679, discloses that the impedance changes which occur in the strip line because of differing wave transmission media can be minimized by adjusting either the ground plane spacing or the width of the center conductor to offset the change in dielectric constant. In FIG. 2, there is shown a strip line Y-junction circulator in which ferrite discs 18 and 19 serve as the wave transmission medium for a part of the strip line and air is the wave transmission medium in the other parts of the line. In accordance with the teaching of my copending application, the characteristic impedance of the strip line is maintained substantially unchanged by offsetting the higher dielectric constant of the ferrite filled portion of the strip line through a reduction in the width of the line's center conductor. Referring to the plan view of the junction, depicted in FIG. 3, it can be seen that the width of the center conductor 20 in the air filled portion of the line is W whereas in the ferrite filled part of the line, the center conductors width is reduced to W, to match the characteristic impedance of the ferrite filled line to the characteristic impedance of the air filled line. In the strip line Y-junction circulator of FIG. 4, instead of reducing the width of the strip lines center conductor, the ground plane spacing is increased in the ferrite filled portion of the line. That is, the width W of the strip line's center conductor 21 is maintained unchanged and the higher dielectric constant of ferrite discs 22 and 23 is offset by an increase in ground plane spacing b. The increase in ground plane spacing is achieved by providing bores, such as the bore 24, in ground plane plate 25 and 26 and placing the ferrite discs in the bores so that they fill the bores and rest upon the center conductor. By adjusting the ground plane spacing or the width of the center conductor or both, the characteristic impedance of the strip line can be maintained substantially constant. Because no external impedance matching devices, such as dielectric rings are used, the strip line in FIGS. 2 and 4 are said to be inherently matched.

The inherently matched strip line Y-junction circulators operate effectively over a broader frequency band than the conventional circulator, have greater temperature stability than the conventional device, and their performance over the band width of the conventional device is equal to or better than the performance of the conventional circulator. The invention here disclosed rests upon the discovery that further improvement upon the inherently matched circulators can be obtained by placing tuning stubs at the junction.

In FIG. 5, a circulator is shown which utilizes tuning stubs and which, in essence, is an improvement upon the FIG. 2 device. The ports, 30, 31, 32 of the FIG. circulator are coaxial connectors whose inner conductors are connected to the arms 33, 34, 35 of a strip transmission line formed by ground plane plates 36, 37 and the center conductor 38. The junction and a portion of the arms 33, 34, 35 of the center conductor are sandwiched between a pair of ferrite discs 39 and 40. The diameter of the discs is approximately a half wavelength of the longest wavelength in the circulators operating range. Tuning stubs 41, 42, 43 which are entirely within the area covered by the ferrite discs, protrude from the junction. A plan view of the junction is depicted in FIG. 6 to more distinctly show the arrangement of the tuning stubs. The arms 33, 34, 35 of the center conductor extend over the ferrite disc 39 to form a juncture at the discs geometrical center. Within the region of the ferrite disc, the arms have a width W,, but where the arms extend beyond the disc, the width W, is enlarged because those portions of the line are air filled and therefore have a lower dielectric constant for the wave transmission medium than the ferrite filled part of the line.

The tuning stub 41 is disposed between arms 33 and 34, tuning stub 42 is between arms 34 and 35, and tuning stub 43 is located between arms 35 and 33. Assuming that the magnetic field in which the ferrite discs are situated is in a direction causing a signal applied at port 30 to be transmitted to port 31 the input impedance appearing at port 30 can be adjusted by altering the length of tuning stub 41. Similarly, the impedance appearing at port 31 can be adjusted by altering the length of tuning stub 42, and the impedance appearing at port 32 can be tuned by adjustment in the length of stub 43. Each port can have its impedance adjusted independently of the other ports by tuning the stub located between that port and the next adjacent port. Each tuning stub at the junction is preferably located midway between a pair of arms to preserve the junctions symmetry. The tuning stub permits full control of the complex impedance; that is, the tuning stub permits adjustment of the reactive as well as the resistive component of impedance. Stated differently, the magnitude and phase of the admittance presented to the junction is determined by the configuration of the stub and principally by its width and length. The width W, of the stub has a relationship to the width W,of the arms of the center conductor; it has been determined empirically that the stub width W, should be about one-half of the arm width W, It was found that where the width W,, of the center conductor arm corresponded to an impedance of 50 ohms, optimum performance of the circulator was obtained when the width W, was reduced to give a characteristic impedance of 38 ohms and the width W, of the stub produced a characteristic impedance of 64 ohms. That is, optimum performance was obtained when the ferrite filled line was deliberately mismatched to the air filled line and the stub length L was adjusted to provide the appropriate impedance at the input port. In FIG. 6, L designates the length of the tuning stub measured from the center of the junction of the center conductor arms to the tip of the stub.

The contours of constant radio frequency electric field intensity occurring within the area of the circulators ferrite disc is shown in FIG. 7 to illustrate the non-reciprocal field displacement principle of the circulators operation. The depicted field arises when a signal, incident upon port 33 is transmitted to port 34. The plane wave signal incident upon the input port is distorted within the ferrite region so that field intensities increase toward the zone between input port 33 and output port 34 as indicated by the relative values of the contour lines in FIG. 7. The tuning stub 41, located midway between ports 33 and port 34, is situated where the intensity of the field is greatest whereas stubs 42 and 43 are located where the field intensity is low. Because stubs 42 and 43 are situated in low intensity zones of the radio frequency electric field, they have negligible effect upon the signal transmission from port 33 to port 34. It is for that reason that port 33 can be independently tuned by adjusting the length of stub 41. Adjustment of the length of tuning stub 41 determines the protrusion of that stub into the peak intensity zone of the field and permits the optimum transfer of the signal from input port 33 to output port 34 to be obtained. It is evident that the configuration of stub 41 can be changed, if desired, but the rectangularly shaped stub facilitates tuning because only its length need be altered.

A stub tuned circulator is depicted in FIG. 8 employing a pair of ground plane plates 44 and 45 which are held in parallel separated positions by spacers 46, 47, and 48. Each ground plane plate has a cylindrical recess, such as the cavity 49, which effectively increases the spacing between the parallel ground plane plates. The cylindrical recesses are filled by a pair of ferrite discs 50 and 51 which rest upon a center conductor 52 having three arms 53, 54, 55 arranged symmetrically about their junction. Each of the arms is connected to the inner conductor of a coaxial connector 56, 57, 58 which have their outer conductors secured in the spacers. At the junction of the arms are tuning stubs, 59, 60 and 61, situated so that each stub is midway between a pair of arms. The diameter of the ferrite discs is approximately a half wavelength of the lowest frequency in the circulators operational range. As the tuning stubs, measured from the center of the junction are less than a quarter wavelength long, the tuning stubs are contained within the area of the ferrite discs. The increase in ground plane spacing permits the characteristic impedance of the ferrite filled portion to be maintained within that impedance range where the tuning stubs are effective. Where the air filled portion of the line has a characteristic impedance of 50 ohms, it was detennined that optimum performance was obtained when the increase in ground plane spacing gave the ferrite filled line a characteristic impedance of 38 ohms and width W, of the stub produced a characteristic impedance of 64 ohms. The ratio of the characteristic impedance of the ferrite filled line to the characteristic impedance of the tuning stub remains the same whether an increase in ground plane spacing or a change in the width W, of the center conductor is employed to adjust the characteristic impedance of the ferrite filled strip line.

It has been determined that the effect of the tuning stubs on the input impedance of the center conductor arms seen at the face of the ferrite is determined in large measure by the ratio of the characteristic impedance of the tuning stubs to the characteristic impedance of the ferrite-filled line. With a rectangular tuning stub, that ratio of characteristic impedances controls the amplitude of the tuning effected by the stub and the length of the stub determines the phase of the tuning. The precise behavior of the tuning stubs cannot be fully predicted because of the electric field distribution within the ferrite junction area of the circulator. It is believed that the shape of the tuning stub in the region of the high intensity electric field alters the electric field distribution and thereby efiects a tuning" of the center conductor arm.

Although several embodiments of the invention have been here illustrated and described, it is apparent that the invention can take other forms. For example, while the junctions have been illustrated as having ferrite discs, it is apparent that the ferrite may be triangular in form or may take other configurations that preserve the symmetry of the junction. Further, it is apparent to those familiar with ferrite circulators that the circulator may employ four or more arms rather than the three that are illustrated. Since the invention may be embodied in varied structures, it is not intended that the invention be limited to the forms here illustrated or described. Rather, it is intended that the invention be construed to embrace those structures which, in essence, utilize tuning stubs in the manner here disclosed.

I claim:

1. In a circulator of the strip transmission line type in which a plurality of strip line center conductor arms meet at a common junction,

the junction and the contiguous portion of each arm are disposed between a pair of ferrite members situated between the ground plane plates of the strip transmission line, and

means are provided for placing the ferrite members under the influence of a magnetic field, the improvement comprising,

tuning stubs protruding from the junction, each adjacent pair of center conductor arms having a tuning stub disposed between them, the tuning stub extending into a region where a high intensity electric field is established by the electrical energization of the input one of the adjacent arms, and

the characteristic impedance of the tuning stub being greater than the characteristic impedance of the ferritefilled strip transmission line input arm.

2. The improvement according to claim 1, further characterized by each tuning stub being a straight length of line bisecting the angle formed by the pair of arms between which the stub is situated.

3. In a transmission line circulator comprising a TEM junction in a multiport configuration of transmission lines having as a first conductor of each line common ground-member means and as a second conductor of each line an elongated conductor extending in spaced relation from said groundmember means one from each port of said configuration into said junction, for the propagation of electric wave energy from an input one of said ports to an output one of said ports, and

gyromagnetic means located in said junction for circulating such energy when suitably magnetized; the improvement comprising an electrically conductive tuning stub coupled to said second conductors in said junction and extending therefrom between an input conductor and an output conductor into and terminating within a region adjacent said gyromagnetic means where a high intensity electric field is established by electrical energization of said input conductor, for adjusting the complex impedance appearing at said input port.

4. A circulator according to claim 3 in which the characteristic impedance of said tuning stub is greater than the characteristic impedance of said input conductor.

5. A circulator according to claim 3 in which said groundmember means is comprised of two parallel-plate conductors electrically connected together, and said second conductors are located between said plates and joined together to form said junction.

6. A circulator according to claim 3 comprising three ports and three transmission lines in a Y-configuration, in which a tuning stub as claimed in claim 3 is provided between each adjacent pair of said second conductors.

7. A circulator according to claim 3 in which said tuning stub is a conductor bisecting the angle formed by said input and output conductors.

8. A circulator according to claim 7 comprised of strip-type transmission lines having flat second conductors, in which the input conductor has a first width, and the tuning stub has a second width which is substantially half said first width.

9. A circulator according to claim 3 in which said gyromagnetic means extends only part way from said junction to the ports of said configuration, and a dielectric medium having a dielectric constant different from that of said gyromagnetic means intervenes between the latter and said ports, the characteristic impedance of said input conductor in the vicinity of said dielectric medium is greater than the characteristic impedance thereof in the vicinity of said gyromagnetic means, and the characteristic impedance of said tuning stub is greater than the characteristic impedance of said input conductor both in the vicinity of said dielectric medium and in the vicinity of said gyromagnetic means.

10. A circulator according to claim 9 comprised of striptype transmission lines having flat second conductors, in which said dielectric medium is a gas, and the width of said input conductor is greater in said gas than it is adjacent said gyromagnetic means.

11. A circulator according to claim 9 in which said first and second conductors are closer together in the vicinity of said dielectric medium than in the vicinity of said gyromagnetic means.

12. A circulator according to claim 3 comprised of striptype transmission lines in which said gyromagnetic means comprises at least one ferrite disc located between said first and second conductors and having a diameter that is approximately a half wavelength of the lowest frequency of said energy therein at the lowest frequency in the operating range of said circulator, and said tuning stub, measured from the center of said junction, extends along a surface of said disc toward the periphery of said disc.

13. A circulator according to claim 12 in which the characteristic impedance of said tuning stub is greater than the characteristic impedance of said input conductor.

14. In a transmission line circulator comprising a TEM junction in a multiport configuration of transmission lines having as a first conductor of each line a pair of parallel spaced-apart common ground-members and as a second conductor of each line an elongated conductor extending in spaced relation between said ground-members one from each port of said configuration into said junction, for the propagation of electric wave energy from an input one of said ports to an output one of said ports, and gyromagnetic means located in said junction between said second conductors and each of said ground members for circulating such energy when suitably magnetized; the improvement comprising an electrically conductive tuning stub coupled to said second conductors in said junction and extending therefrom between an input conductor and an output conductor into and terminating within a region adjacent said gyromagnetic means where a high intensity electric field is established by electrical energization of said input conductor, for adjusting the complex impedance appearing at said input port.

15. A circulator according to claim 14 comprising three transmission lines in a Y-configuration, in which a tuning stub as claimed in claim 14 is provided between each adjacent pair of said second conductors.

16. A circulator according to claim 14 in which said gyromagnetic means extends only part way from said junction to the ports of said configuration, and a dielectric medium having a dielectric constant different from that of said gyromagnetic means intervenes between the latter and said ports, the characteristic impedance of said input conductor in the vicinity of said dielectric medium is greater than the characteristic impedance thereof in the vicinity of said gyromagnetic means, and the characteristic impedance of said tuning stub is greater than the characteristic impedance of said input conductor both in the vicinity of said dielectric medium and in the vicinity of said gyromagnetic means.

17. A circulator according to claim 16 comprised of striptype transmission lines having flat second conductors, in which said dielectric medium is a gas, and the width of said input conductor is greater in said gas than it is adjacent said gyromagnetic means.

18. A circulator according to claim 16 in which said groundplane members are spaced closer together in the vicinity of said dielectric medium than in the vicinity of said gyromagnetic means.

19. A circulator according to claim 18 in which said gyromagnetic means comprises a pair of ferrite discs having a diameter that is approximately a half wavelength of the lowest frequency of said energy therein at the lowest frequency in the operating range of said circulator, and said tuning stub, measured from the center of said junction, extends between said discs toward the vicinity of the peripheries of said discs. 

1. In a circulator of the strip transmission line type in which a plurality of strip line center conductor arms meet at a common junction, the junction and the contiguous portion of each arm are disposed between a pair of ferrite members situated between the ground plane plates of the strip transmission line, and means are provided for placing the ferrite members under the influence of a magnetic field, the improvement comprising, tuning stubs protruding from the junction, each adjacent pair of center conductor arms having a tuning stub disposed between them, the tuning stub extending into a region where a high intensity electric field is established by the electrical energization of the input one of the adjacent arms, and the characteristic impedance of the tuning stub being greater than the characteristic impedance of the ferrite-filled strip transmission line input arm.
 2. The improvement according to claim 1, further characterized by each tuning stub being a straight length of line bisecting the angle formed by the pair of arms between which the stub is situated.
 3. In a transmission line circulator comprising a TEM junction in a multiport configuration of transmission lines having as a first conductor of each line common ground-member means and as a second conductor of each line an elongated conductor extending in spaced relation from said ground-member means one from each port of said configuration into said junction, for the propagation of electric wave energy from an input one of said ports to an output one of said ports, and gyromagnetic means located in said junction for circulating such energy when suitably magnetized; the improvement comprising an electrically conductive tuning stub coupled to said second conductors in said junction and extending therefrom between an input conductor and an output conductor into and terminating within a region adjacent said gyromagnetic means where a high intensity electric field is established by electrical energization of said input conductor, for adjusting the complex impedance appearing at said input port.
 4. A circulator according to claim 3 in which the characteristic impedance of said tuning stub is greater than the characteristic impedance of said input conductor.
 5. A circulator according to claim 3 in which said ground-member means is comprised of two parallel-plate conductors electrically connected together, and said second conductors are located between said plates and joined together to form said junction.
 6. A circulator according to claim 3 comprising three ports and three transmission lines in a Y-configuration, in which a tuning stub as claimed in claim 3 is provided between each adjacent pair of said second conductors.
 7. A circulator according to claim 3 in which said tuning stub is a conductor bisecting the angle formed by said input and output conductors.
 8. A circulator according to claim 7 comprised of strip-type transmission lines having flat second conductors, in which the input conductor has a first width, and the tuning stub has a second width which is substantially half said first width.
 9. A circulator according to claim 3 in which saiD gyromagnetic means extends only part way from said junction to the ports of said configuration, and a dielectric medium having a dielectric constant different from that of said gyromagnetic means intervenes between the latter and said ports, the characteristic impedance of said input conductor in the vicinity of said dielectric medium is greater than the characteristic impedance thereof in the vicinity of said gyromagnetic means, and the characteristic impedance of said tuning stub is greater than the characteristic impedance of said input conductor both in the vicinity of said dielectric medium and in the vicinity of said gyromagnetic means.
 10. A circulator according to claim 9 comprised of strip-type transmission lines having flat second conductors, in which said dielectric medium is a gas, and the width of said input conductor is greater in said gas than it is adjacent said gyromagnetic means.
 11. A circulator according to claim 9 in which said first and second conductors are closer together in the vicinity of said dielectric medium than in the vicinity of said gyromagnetic means.
 12. A circulator according to claim 3 comprised of strip-type transmission lines in which said gyromagnetic means comprises at least one ferrite disc located between said first and second conductors and having a diameter that is approximately a half wavelength of the lowest frequency of said energy therein at the lowest frequency in the operating range of said circulator, and said tuning stub, measured from the center of said junction, extends along a surface of said disc toward the periphery of said disc.
 13. A circulator according to claim 12 in which the characteristic impedance of said tuning stub is greater than the characteristic impedance of said input conductor.
 14. In a transmission line circulator comprising a TEM junction in a multiport configuration of transmission lines having as a first conductor of each line a pair of parallel spaced-apart common ground-members and as a second conductor of each line an elongated conductor extending in spaced relation between said ground-members one from each port of said configuration into said junction, for the propagation of electric wave energy from an input one of said ports to an output one of said ports, and gyromagnetic means located in said junction between said second conductors and each of said ground members for circulating such energy when suitably magnetized; the improvement comprising an electrically conductive tuning stub coupled to said second conductors in said junction and extending therefrom between an input conductor and an output conductor into and terminating within a region adjacent said gyromagnetic means where a high intensity electric field is established by electrical energization of said input conductor, for adjusting the complex impedance appearing at said input port.
 15. A circulator according to claim 14 comprising three transmission lines in a Y-configuration, in which a tuning stub as claimed in claim 14 is provided between each adjacent pair of said second conductors.
 16. A circulator according to claim 14 in which said gyromagnetic means extends only part way from said junction to the ports of said configuration, and a dielectric medium having a dielectric constant different from that of said gyromagnetic means intervenes between the latter and said ports, the characteristic impedance of said input conductor in the vicinity of said dielectric medium is greater than the characteristic impedance thereof in the vicinity of said gyromagnetic means, and the characteristic impedance of said tuning stub is greater than the characteristic impedance of said input conductor both in the vicinity of said dielectric medium and in the vicinity of said gyromagnetic means.
 17. A circulator according to claim 16 comprised of strip-type transmission lines having flat second conductors, in which said dielectric medium is a gas, and the width of said input conductor is greater in said gas than it is adjacent Said gyromagnetic means.
 18. A circulator according to claim 16 in which said ground-plane members are spaced closer together in the vicinity of said dielectric medium than in the vicinity of said gyromagnetic means.
 19. A circulator according to claim 18 in which said gyromagnetic means comprises a pair of ferrite discs having a diameter that is approximately a half wavelength of the lowest frequency of said energy therein at the lowest frequency in the operating range of said circulator, and said tuning stub, measured from the center of said junction, extends between said discs toward the vicinity of the peripheries of said discs. 